This project will be concerned with studies of the structure, mechanism of action and regulation of three enzymes involved in the synthesis and utilization of the branched-chain amino acids. These are the acetohydroxy acid synthetase, acetohydroxy acid isomeroreductase and isoleucyl-tRNA synthetase. Our aim is to achieve a better understanding of the mechanisms of enzymic catalysis, the relationship of structure to function and the regulatory properties of these enzymes. Our approach will involve enzyme purification, testing of specific pure enantiomers for activity, kinetic analysis, effect of protein modifications on activity and, in the case of yeast isoleucyl-tRNA synthetase, a comparison of wild type and mutationally altered enzymes.